Public polls showed that Specter had the support of only 30 percent of likely GOP voters in the 2010 primary. At least 180,000 party moderates – his core supporters – switched parties in 2008 to vote for Mr. Obama.

In a press briefing Tuesday, Specter described his prospects for reelection as a Republican as “bleak.” “I am not prepared to have my 29-year record in the United States Senate decided by the Pennsylvania Republican primary electorate,” he said.

A maverick Republican

During his years in the Senate, Specter had plenty of opportunity to rile GOP voters and colleagues with his votes in favor of abortion rights, embryonic stem cell research, and social spending. He also angered conservatives with his role in the 1987 defeat of Robert Bork’s nomination to the Supreme Court. More recently, he blasted “executive branch excesses” during the Bush years in an April 16 essay in The New York Review of Books.

The essay ends with this note: “I doubt that the Democratic majority, which was so eager to decry expansions of executive authority under President Bush, will still be as interested in the problem with a Democratic president in office. I will continue the fight whatever happens.”

But he went too far for GOP conservatives with his Feb. 13 vote for Obama’s $787 billion stimulus package.

Talks with Specter had been going on for years, Reid told reporters Tuesday. “I had a long dialogue with Senator Specter about his place in the evolving Republican Party,” he said.

The 60th vote?

Specter’s shift takes Democrats within one vote of the 60-vote threshold for cutting off a filibuster, likely giving Democrats a working majority on key procedural votes for the first time in the 111th Congress. This could prove immensely important in ensuring that high-profile elements of Obama's agenda – such as healthcare reform – come to a vote.

The deal that Reid offered Specter was a guarantee that his seniority as a Republican would carry over to his ranking as a Democrat. That means that Specter is positioned to unseat Sen. Tom Harkin (D) as chair of the powerful appropriations subcommittee that oversees labor, health, and education – as early as the next Congress.

“Senator Specter knows that no one will be dumped off the full committee or a subcommittee. It’s a voluntary process,” Reid said. “In a year and a half, it’s a new game.

“This is not a time to gloat or to give high fives,” he added. “I learned early on that every vote counts.”

In an age of highly polarized politics, centrists often face questions about their interest in switching parties. Sen. Ben Nelson (D) of Nebraska, long viewed by Republicans as a prospect to switch to their side of the aisle, says that he’s “very happy” where he is.

“My party has not left me, but this is a town where everything is possible, so never say never,” he said Tuesday.

Republicans say the Specter defection puts more pressure on Democrats who ran as moderates to vote that way.

“I think there’ll be enormous pressure … on red-state Democrats who ran as moderates to actually vote as moderates. To the extent that they give a left-leaning agenda a blank check, they will be voting differently from the way they ran,” says Senate Republican leader Mitch McConnell.

Since 1890, 20 senators have switched parties. At the turn of the last century, the issues most often behind partisan rebranding were the agrarian politics and the demonetization of silver. The defection of Sen. Strom Thurmond of South Carolina to the Republicans in 1964 marked the start of a regional realignment over civil rights that bolstered conservative GOP ranks.

Specter describes his own exit, and the depletion of the ranks of GOP moderates, as a consequence of that realignment.

“As the Republican Party has moved farther and farther to the right, I have found myself increasingly at odds with the Republican philosophy and more in line with the philosophy of the Democratic Party,” he said.

“When the stimulus package came up for a vote, I felt that it was indispensable to vote aye in order to avoid the possibility of a 1929-type depression,” he added.

Despite differences, GOP leaders often rallied to support Specter in past tight elections. Before the 2006 primary, President Bush flew with him to Philadelphia on Air Force One and praised Specter as an “ally.”

Sen. Thad Cochran (R) of Mississippi campaigned with Specter beginning with this first race. “I asked him at the caucus lunch: Does this mean I don’t have to go to Erie [Pa.] anymore?”